Petunia × Calibrachoa 'Light Yellow' (× Petchoa 'Light Yellow') is a kind of perennial herbaceous flower obtained through intergeneric hybridization of Petunia and Calibrachoa with high ornamental value and wide application, facing challenges in seed acquisition. Expanding propagation through tissue culture is an economically efficient means. Hence, establishing an effective procedure for the storage of callus is essential for × Petchoa 'Light Yellow'. Cryopreservation is an effective method for the in vitro propagation and long-term preservation of × Petchoa 'Light Yellow' germplasms. For formulating the optimization of the vitrification procedure, first, an orthogonal experimental design was employed to pinpoint critical steps in the vitrification protocol (pre-culture, osmoprotection, dehydration, and dilution) for Petunia × Calibrachoa callus tissues and then five additional factors (pre-culture, osmoprotection I and II, dehydration, and dilution) were optimized to further reduce the sample water content and enhance cell viability levels. The vitrification procedure was described as follows: callus tissues were precultured in MS solid medium with 0.3 M sucrose for 5 d, incubated with osmoprotection solution I and II for 15 min at 25 °C, respectively, cryoprotected with PVS2 for 30 min at 0 °C, and rapidly immersed in liquid nitrogen. Cryopreserved callus tissues were then diluted in MS liquid medium with 1.2 M sucrose for 20 min at 25 °C and recovered on MS solid medium with 0.5 mg/L 6-BA and 0.1 mg/L NAA, and sucrose. The cell viability measured by TTC staining was approximately 16 %-18 % after 72 h-recovery. Following 45 days, the relative survival of callus reached up to 49.48 %. Furthermore, EST-SSR analysis showed no significant difference in the genetic stability of cryopreserved callus compared to the control. Based on the cryopreservation of × Petchoa 'Light Yellow' callus, we further evaluated the response of callus water contents to the osmotic stress in the optimized and original protocols (CK) for a higher cryopreservation survival. A comparative analysis of water content demonstrated that the procedure of gradual and gentle dehydration significantly improved water content and cell survival. Ultrastructural changes between cryopreserved and non-cryopreserved callus were examined and high vacuolation emerged as a key determinant, indicating its substantial impact on the low survival of cryopreserved cells, which should help us to understand the effectiveness of osmotic protectants in dehydration.
Keywords: Callus tissues; Cryopreservation; Dehydration; Petunia × Calibrachoa; Vacuolation; Water content.
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